Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may be intermittently diagnosed for the presence of undesired emissions that could release fuel vapors to the atmosphere. Undesired evaporative emissions may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system and/or an emissions control system may be isolated at an engine-off event. The pressure in such a fuel system and/or an emissions control system will increase if the tank is heated further (e.g., from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and undesired emissions identified based on expected vacuum development or expected rates of vacuum development. However, the entry conditions and thresholds for a typical EONV test may be based on an inferred total amount of heat rejected into the fuel tank during the prior drive cycle. The inferred amount of heat may be based on engine run-time, integrated mass air flow, miles driven, etc. If these conditions are not met, the entry into the evaporative emissions test is aborted. Thus, hybrid electric vehicles, including plug-in hybrid electric vehicles (HEV's or PHEV's), pose a problem for effectively controlling evaporative emissions. For example, primary power in a hybrid vehicle may be provided by the electric motor, resulting in an operating profile in which the engine is run only for short periods. As such, adequate heat rejection to the fuel tank may not be available for EONV diagnostics.
An alternative to relying on inferred sufficient heat rejection for entry into an EONV diagnostic test is to instead actively pressurize or evacuate the fuel system and/or emissions control system via an external source. For example, a method may perform a pressure-based evaporative emissions test using a pump to pressurize and/or evacuate the fuel system and/or emissions control system. The fuel system and/or evaporative emissions control system may then be monitored for a selected time period, and if the pressure falls below a threshold value if initially pressurized, or rises above a threshold value if initially evacuated, the system identifies undesired emissions. As such, by conducting evaporative emissions tests via the use of an external pressure source, reliance on heat rejected from the engine may be circumvented.
Whether relying on EONV or actively pressurizing or evacuating the fuel system and evaporative emissions control system, the entire fuel system and evaporative emissions control system must be diagnosed for potential undesired emissions. This includes the cap or capless area and the entire vapor space of the fuel tank. However, certain parking conditions may prevent the testing of the entire fuel system and/or evaporative emissions control system for undesired emissions. For example, when parking on steep slopes, liquid fuel can shut closed certain passive fuel tank valves thus restricting communication between the fuel tank and the rest of the evaporative emissions control system. Other potential problems resulting from parking on grades may include the formation of an isolated vapor dome space that is not in communication with the rest of the fuel system and evaporative emissions control system. For example, due to packaging constraints, fuel tank geometries may have many cavities wherein some areas may be higher than others, such that when parking on an incline isolated vapor dome spaces may result. Any areas of the fuel system and/or evaporative emissions control system that go unchecked as a result of parking on an incline violates regulatory requirements for evaporative emissions testing.
The fact that liquid fuel may result in isolated vapor spaces and a restriction of communication between the fuel tank and the rest of the fuel system and evaporative emissions control system when the vehicle is inclined has been described. For example, US Patent No. US 20140069394 teaches conducting an engine-on evaporative emissions test, and responsive to an unintended closing of a fuel tank vent valve, for example due to vehicle travel along an incline that is higher than a threshold grade, discontinuing the evaporative emissions test and resuming the test at a later time. However, the inventors herein have recognized potential issues with such a method. For example, the method does not teach mitigating action for an engine-off evaporative emissions test where communication between the fuel tank and the rest of the fuel system and evaporative emissions control system is restricted and/or one or more isolated vapor dome space(s) is created due to the vehicle being parked on a steep slope.
Thus, the inventors herein have developed systems and methods to at least partially address the above issues. In one example, a method is provided comprising, responsive to a first vehicle-off condition, maintaining a vehicle compound angle with respect to ground and conducting an evaporative emissions test, and responsive to a second vehicle-off condition, leveling the vehicle a determined amount and then conducting the evaporative emissions test.
As one example, responsive to the vehicle-off condition, a fuel level and a vehicle compound angle are indicated, wherein indicating the vehicle compound includes indicating a vehicle pitch angle and a vehicle bank (roll) angle. Based on the fuel level and vehicle compound angle, it may be determined whether the fuel level and vehicle compound angle is above a predetermined threshold, thus resulting in fuel in the fuel tank causing the closing of one or more fuel tank vent valves, and/or causing the formation of an isolated fuel tank vapor dome(s) resulting from one or more section(s) of the fuel tank being isolated from any other section(s) of the fuel tank. In other words, it may be determined whether the fuel level and vehicle compound angle is causing fuel system isolation issues that may impact the results of an evaporative emissions test. Determining whether the combined fuel level and vehicle parking condition is above a predetermined threshold may be based on computer aided design modeling of the fuel tank. As such, in the first condition, it may be determined that the fuel level and vehicle compound angle is below a threshold, thus an evaporative emissions test procedure may be conducted without prior leveling of the vehicle. Alternatively, in the second condition, it may be determined that the fuel level and vehicle compound angle is above a threshold, thus the vehicle may be leveled prior to conducting the evaporative emissions test procedure.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.